In the past, non-isolated, step-down, direct current converters using buck topology were used because they included only a single inductor for their magnetics and no transformer. Using the simplest possible magnetics is important because the cost of the magnetics rises rapidly as additional windings are added. The cost not only increases due to manufacturing cost but also due to testing cost. Further, more complex magnetics increase the chances of errors in construction, which may lead to failures of the converter. However, these buck converters were capable of producing only a single output voltage. If more than one output voltage was required, either a linear regulator at the output of the buck converter, a second buck converter at the output of the first converter, or a parallel buck converter was required. Using a linear regulator, although low cost, involves very poor efficiency and concomitant heat problems. Use of a second buck converter or a parallel buck converter both involve adding one or more additional complete converters, with all of their additional magnetics, controller integrated circuits, etc. Further, the series connection of two frequency converters may halve the efficiency of the system.
Many systems require multiple low voltage power buses for operation. In particular, computer motherboards need large currents at, for example, both 3.3 volts and 2.0 volts. A conventional approach for generating these two voltages is to have a separate buck converter for each output. Each converter involves a control integrated circuit, two MOSFETs (for synchronous conversion), an inductor, one or more output capacitors, and a variety of small signal components. These numerous parts, and inductors in particular, make power conversion very expensive. A simpler and less costly solution has long been sought.